The present invention relates to an oxygen-cathode used in a process for electrolysis of an alkali chloride, particularly in a process for electrolysis of the alkali chloride while using an ion-exchange membrane or a process for electrolysis of the alkali chloride according to SPE method, wherein a cathode reaction is caused by supplying oxygen or an oxygen-containing gas such as air into the inner part of the cathode, thereby carrying out the electrolysis of an aqueous solution of the alkali chloride without generating hydrogen in the cathode side in an electrolytic cell.
In recent years, in the industry of alkali chloride electrolysis, the diaphragm method has come to be used instead of the mercury electrode method in view of preventing the environmental pollution, and further, the ion-exchange membrane method has come to be more used in order to obtain sodium hydroxide at a higher purity and in a higher concentration. Also the SPE method has been developed for carrying out electrolysis under a voltage as low as possible while eliminating the ohmic loss due to the aqueous electrolyte solution.
In order to effect the electrolysis at a still lower voltage in the above-mentioned ion-exchange membrane method or in the SPE method, various studies have been carried out, and particularly concerning the cathode used in the electrolysis, it has been recently known that in the case where oxygen or an oxygen-containing gas such as air is supplied to the cathode side from outside of the cell, the gas diffuses into the cathode, the oxygen reacts with water in an aqueous solution in the vicinity of the cathode to form hydroxide ions thus preventing the generation of hydrogen in the cathode side and accordingly, the electrolysis can be effected at a lower voltage than that of the usual case without generating a hydrogen gas in the vicinity of the cathode.
The cathode used in the recently known method is called as the "oxygen-cathode", and must have a specified construction by which the supplied gas diffuses into the internal part of the cathode, and it is still required that the cathode is excellent in the cathode specificity and also in durability.
In addition, a device is also necessary for preventing the leakage of the aqueous solution present in the vicinity of the cathode through the gas-permeable cathode.
Hitherto, as the above-mentioned oxygen-cathodes, (1) an oxygen-cathode produced by mixing a hydrophobic material such as particles of polytetrafluoroethylene with a catalytic substance, hardening the mixture and adhering closely the thus hardened product to the current collector such as a nickel grid, a reticulated material or a porous material, (2) an oxygen-cathode produced by impregnating a porous, sintered alloy with the catalytic substance and subjecting the thus impregnated alloy to the hydrophobic treatment with polytetrafluoroethylene (for instance, refer to Japanese Patent Application Laying-Open No. 54-97600(1979)) or 3) an oxygen-cathode having an improved cathode-performance by by adding a specific pore-forming agent to the baked material of the blended mixture of the catalytic substance and a hydrophobic substance (for instance, refer to Japanese Patent Application No. 55-28216 (1970)) have been known, however, in the case where the sintered alloy is used as the micropore layer, there are demerits that it is difficult to control the distribution of the pore size and the thin alloy material is apt to be broken. In the case where the active layer of the cathode is made by hardening particles of polytetrafluoroethylene, etc., the thus prepared cathode is still insufficient in its performance and there is a problem in joining with the current collector. In either case, contact resistance is caused between the electrode substrate and the current collector and accordingly, it is impossible to obtain any product having a sufficient performance as the oxygen-cathode for use in electrolysis of an alkali chloride.
In consideration of the above-mentioned problems, the object of the present invention is to provide an improvement of the cathode substrate to be used in electrolysis of an alkali chloride and solution of the problems on the joining-ability of the cathode substrate and the current collector. As a result of studying the problem, inventors have found the object of the present invention has been achieved by (1) preparing the cathode substrate by using an anticorrosive and durable material in which the control of porosity and pore size is easily carried out and (2) joining the cathode substrate with the current collector into a unified body, thereby reducing the contact resistance as small as possible.